This invention relates to a fluidized bed reactor and more particularly, to a method and system for eliminating gas backflow in the solids recycle system of a fluidized bed reactor.
Fluidized bed reactors, such as gasifiers, steam generators, combustors, and the like, are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. The entrained particulate solids are separated externally of the bed and recycled back into the bed. The heat produced by the fluidized bed is utilized in various applications which results in an attractive combination of high heat release, high sulfur absorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed reactor is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well-defined, or discrete, upper surface.
In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turndown afforded by the bubbling bed, a fluidized bed reactor has been developed utilizing a "circulating" fluidized bed process. According to this process, the fluidized bed density is well below that of a typical bubbling fluidized bed, the air velocity is greater than that of a bubbling bed and the flue gases passing through the bed entrain a substantial amount of particulate solids and are substantially saturated therewith.
Also, the circulating fluidized bed is characterized by relatively high solids recycling which makes it insensitive to fuel heat release patterns, thus minimizing temperature variations and, therefore, decreasing the nitrogen oxides formation. Also, the high solids recycling improves the efficiency of the mechanical device used to separate the gas from the solids for ensuring high solids recycle. The resulting increase in sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption. To accomplish this it is necessary to have a loop seal device that does not restrict the flow of solids or reduce collection efficiency. In other situations or designs, such as for high ash fuels, it may be necessary to effect the collection efficiency by passing air/gas flow upwards to a dip leg extending from the separator device.
Most of the circulating bed designs currently being utilized control load by regulating the solids recycle rate and some approach this by reducing the solids inventory from the loop seal, i.e., from the sealing system located between the outlet of the external separating devices and the recycle inlet to the fluidized bed. However, this normally has to be accomplished with a metering cooler, such as a water cooled screw to remove solids from the recycle system, which adds mechanical complexity and costs penalties in addition to requiring downstream handling equipment.
In U.S. Pat. No. 4,781,574 issued Nov. 1, 1988, and assigned to the assignee of the present invention, the above problems were addressed by disposing an air source at the separated solids outlet of a cyclone separator and discharging air into the separator in a direction opposite the direction of flow of the separated solids. The air entrains a portion of the solids and is passed back through the separator and to the heat recovery area Although this technqiue enabled the solids inventory to be controlled without incurring significant additional costs, it interfered with the operation of the separator.